This invention relates to techniques for sealing structures, particularly flat-panel devices.
A flat-panel device contains a pair of generally flat plates connected together through an intermediate mechanism. The two plates are typically rectangular in shape. The thickness of the relatively flat-structure formed with the two plates and the intermediate connecting mechanism is small compared to the diagonal length of either plate.
When used for displaying information, a flat-panel device is typically referred to as a flat-panel display. The two plates in a flat-panel display are commonly termed the faceplate (or frontplate) and the baseplate (or backplate). The faceplate, which provides the viewing surface for the information, is part of a faceplate structure containing one or more layers formed over the faceplate. The baseplate is similarly part of a baseplate structure containing one or more layers formed over the baseplate. The faceplate structure and the baseplate structure are sealed together, typically through an outer wall, to form a sealed enclosure.
A flat-panel display utilizes mechanisms such as cathode rays (electrons), plasmas, and liquid crystals to display information on the faceplate. Flat-panel displays that employ these three mechanisms are generally referred to as cathode-ray tube (xe2x80x9cCRTxe2x80x9d) displays, plasma displays, and liquid-crystal displays. The constituency and arrangement of the display""s faceplate structure and baseplate structure depend on the type of mechanism utilized to display information on the faceplate.
In a flat-panel CRT display, electron-emissive elements are typically provided over the interior surface of the baseplate. The electron-emissive elements are arranged in a matrix of rows and columns of picture elements (pixels). Each pixel typically contains a large number of individual electron-emissive elements. When the electron-emissive elements are appropriately excited, they emit electrons that strike phosphors arranged in corresponding pixels situated over the interior surface of the faceplate.
The faceplate in a flat-panel CRT display consists of a transparent material such as glass. Upon being struck by electrons emitted from the electron-emissive elements, the phosphors situated over the interior surface of the faceplate emit light visible on the exterior surface of the faceplate. By appropriately controlling the electron flow from the baseplate structure to the faceplate structure, a suitable image is displayed on the faceplate.
The electron-emissive elements in a flat-panel CRT display typically emit electrons according to a field-emission (cold emission) technique or a thermionic emission technique. In either case, but especially for the field-emission technique, electron emission needs to occur in a highly evacuated environment for the CRT display to operate properly and to avoid rapid degradation in performance. The enclosure formed by the faceplate structure, the baseplate structure, and the outer wall is thus fabricated in such a manner as to be at a high vacuum, typically a pressure of 10xe2x88x927 torr or less for a flat-panel CRT display of the field-emission type. One or more spacers are commonly situated between the faceplate structure and the baseplate structure to prevent outside forces, such as air pressure, from collapsing the display.
Any degradation of the vacuum can lead to various problems such as non-uniform brightness of the display caused by contaminant gases that degrade the electron-emissive elements. The contaminant gases can, for example, come from the phosphors. Degradation of the electron-emissive elements also reduces the working life of the display. It is thus critical to hermetically seal a flat-panel CRT display.
A flat-panel CRT display of the field-emission type, often referred to as a field-emission display (xe2x80x9cFEDxe2x80x9d), is conventionally sealed in air and then evacuated through pump-out tubulation provided on the display. FIGS. 1a-1d (collectively xe2x80x9cFIG. 1xe2x80x9d) illustrate one such conventional procedure for sealing an FED consisting of a baseplate structure 10, a faceplate structure 12, an outer wall 14, and multiple spacer walls 16.
At the point shown in FIG. 1a, spacer walls 16 are mounted on the interior surface of faceplate structure 12, and outer wall 14 is connected to the interior surface of faceplate structure 12 through frit (sealing glass) 18 provided along the faceplate edge of outer wall 14. Frit 20 is situated along the baseplate edge of outer wall 14. A tube 22 is sealed to the exterior surface of baseplate structure 10 through frit 24 at an opening 26 in baseplate structure 10. A getter 28 for collecting contaminant gases is typically provided along the inside of tube 22. The structure formed with baseplate structure 12, outer wall 14, and spacer 16 is physically separate from the structure formed with baseplate structure 10, tube 22, and getter 28 prior to sealing the display.
Structures 12/14/16 and 10/22/28 are placed in an alignment fixture 30, aligned to each other, and brought into physical contact along frit 20 as shown in FIG. 1b. Alignment fixture 30 is located in, or is placed in, an oven 32. After being aligned and brought into contact, structures 12/14/16 and 10/22/28 are slowly heated to a sealing temperature ranging from 450xc2x0 C. to greater than 600xc2x0 C. Frit 20 melts, sealing structure 12/14/16 to structure 10/22/28. The sealed FED is slowly cooled down to room temperature. The heating/sealing/cool-down process typically takes 1 hr.
After having been sealed, the FED is removed from alignment fixture 30 and oven 32, and is placed in another oven 34. See FIG. 1c. A vacuum pumping system 36 is connected to tube 22. With a heating element 38 placed around tube 22, the FED is pumped down to a high vacuum level through tube 22. The FED is then brought slowly up to a high temperature and baked for several hours to remove contaminant gases from the material of the FED. When a suitable low pressure can be maintained in the FED at the elevated temperature, the FED is cooled to room temperature, and tube 22 is heated through heating element 38 until tube 22 closes to seal the FED at a high vacuum. The FED is then removed from oven 34 and disconnected from vacuum pump 36. FIG. 1d shows the sealed FED.
The sealing process of FIG. 1 is unsatisfactory for a number of reasons. Even though multiple FEDs can be sealed at the same time, the sealing procedure often takes too long to meet commercial needs. In addition, the entire FED is heated to a high temperature for a long period. This creates concerns relating to alignment tolerances and can degrade certain of the materials in the FED, sometimes leading to cracking. Furthermore, tube 22 protrudes out of the FED. Consequently, the FED must be handled very carefully to avoid breaking tube 22 and destroying the FED. It would be extremely beneficial to have a technique for sealing a flat-panel device, especially a flat-panel display of the field-emission CRT type, that overcomes the foregoing problems and eliminates the need for pump-out tubulation such as tube 22.
The present invention furnishes a technique for sealing portions of a structure together in such a manner that the sealed structure can readily achieve a reduced pressure state, typically a high vacuum level, without the necessity for providing the structure with an awkward pressure-reduction device, such as pump-out tubulation, that protrudes substantially beyond the remainder of the sealed structure. In the invention, sealing is effected by a gap-jumping technique in which energy is applied locally along a specified area to create the seal. The term xe2x80x9clocalxe2x80x9d or xe2x80x9clocallyxe2x80x9d as used here in describing an energy transfer means that the energy is directed selectively to certain material largely intended to receive the energy without being significantly transferred to nearby material not intended to receive the energy.
In using the gap-jumping technique of the invention to seal a structure, the entire structure is typically heated prior to completing the seal in order to drive out contaminant gases and alleviate stress that might otherwise arise during completion of the seal. However, the maximum temperature reached during the outgassing/stress-relieving operation, typically in the vicinity of 300xc2x0 C., is much less than that normally reached in prior art sealing processes such as that described above in which sealing is performed by global heating. Problems such as cracking and degradation of the components of the structure are greatly reduced with the present gap-jumping sealing technique.
The sealing technique of the invention can be performed in much less time than a prior art sealing process of the type described above. The present sealing technique is particularly suitable for sealing a flat-panel device, especially a flat-panel display of the CRT type. With the necessity for awkwardly protruding pump-out tubulatlion eliminated, the possibility of destroying the sealed structure by breaking a pump-out tube is avoided. In short, the invention provides a large advantage over prior art hermetic sealing techniques.
Broadly, the sealing technique of the invention involves positioning a sealing area of one body near a matching sealing area of another body such that a gap at least partially separates the two sealing areas. The gap typically has an average height of at least 25 xcexcm.
In one implementation of the present sealing technique, a pair of local energy transfers are now performed. Specifically, energy is initially transferred locally to material of a specified one of the bodies along part of the gap while the bodies are in a non-vacuum environment. The initial local energy transfer causes material of the bodies to bridge that part of the gap and partially seal the two bodies together along the sealing areas. Energy is subsequently transferred locally to material of the specified body along the remainder of the gap while the bodies are in a vacuum environment, normally a high vacuum. The subsequent local energy transfer causes material of the bodies to bridge the remainder of the gap and complete the sealing of the two bodies together.
The local energy that causes the gap to be bridged (or jumped) is typically light energy, preferably furnished by a laser in at least one of the energy-transferring steps. Alternatively, a focused lamp can furnish the light energy. Also, at least one of the energy-transferring steps can be performed with another type of local energy such as locally directed radio-frequency (xe2x80x9cRFxe2x80x9d) wave energy, including microwave energy. In a typical case, the material of the specified bodyxe2x80x94i.e., the body that receives the local energy in both the initial non-vacuum energy-transferring step and the subsequent vacuum energy-transferring stepxe2x80x94bridges largely all of the gap.
Depending on the geometry of the structure to be sealed, on the materials used in the structure, and on the conditions of the local energy transfers, one or more of several mechanisms appear to be responsible for gap jumping in the present invention. One mechanism is surface tension. As energy is locally transferred to the specified body along its sealing area at the gap between the two bodies, the material along the sealing area of the specified body melts and, especially if the sealing area is relatively flat up to a pair of corners, attempts to occupy a volume having a reduced surface area. This causes material of the specified body along its sealing area to curve towards the sealing area of the other body.
Gases trapped in the material of the specified body near its sealing area, or created by changes in the composition of the material of the specified body along its sealing area, may help cause material of the sealing area of the specified body to move towards the other sealing area. Also, in some cases, the material of the specified body along its sealing area may undergo a phase change that results in a decrease in density so that the volume of the material increases, causing it to expand towards the other sealing area.
In any event, the molten material of the specified body along its sealing area comes into contact with the material of the other body along its sealing area, wets that material, and flows to form a seal. The net result is that application of local energy to the sealing area of the specified body causes part of its material to close the gap between the two sealing areas. The gap must, of course, be sufficiently small so as to be capable of being bridged due to the local energy transfer. We have successfully jumped gaps of up to 300 xcexcm utilizing local light energy transfer in accordance with the invention.
Use of a non-vacuum environment followed by a high vacuum environment to perform the energy-transferring steps yields a number of benefits. Performing the initial local energy transfer in a non-vacuum environment to bridge part of the gap normally enables the material that bridges that part of the gap to have a lower porosity, and thus a higher density, than otherwise identical material subjected to the same type of local energy transfer but in a high vacuum. When the non-vacuum environment consists largely of nitrogen (a relatively non-reactive gas) or/and an inert gas during at least part of the initial local energy transfer, the number of undesired chemical reactions that occur between gases in the non-vacuum environment and the materials being sealed is greatly reduced. The net result is that a strong seal is formed with the material that bridges part of the gap during the initial energy-transferring step.
With the sealed structure forming an enclosure at the end of the subsequent energy-transferring step, performing the subsequent local energy transfer in a high vacuum environment to bridge the remainder of the gap and finish the seal results in a high vacuum being created in the enclosure. Importantly, the vacuum is produced in the enclosure during the end of the sealing procedure without using a device such as a pump-out tube to create the vacuum. The combination of a non-vacuum environment for the initial local energy transfer and a high vacuum environment for the subsequent local energy transfer thereby enables a strong hermetic seal to be made between the two bodies while avoiding the necessity of using pump-out tubulation to produce a high vacuum in the sealed enclosure.
When used in sealing a structure such as a flat-panel display, the sealing technique of the invention entails positioning a first edge of a primary wall (e.g., an outer wall) near a matching sealing area of a first plate structure (e.g., a baseplate structure) such that a gap at least partially separates the first edge of the wall from the sealing area of the first plate structure. Energy is then transferred locally to material of the wall along the gap to produce gap jumping that closes the gap. The local energy transfer is typically performed by the composite non-vacuum/vacuum approach described above.
A second edge of the wall opposite the first edge is usually sealed (or joined) to a second plate structure (e.g., a faceplate structure) along another matching sealing area. Sealing of the second plate structure to the wall is typically done in a non-vacuum environment before sealing the first plate structure to the wall. However, sealing of the second plate structure to the wall can be performed at the same time that the first plate structure is sealed to the wall utilizing, for example, a double-laser technique. In either case, the gap-jumping seal of the first plate structure to the wall is typically completed in a vacuum environment, again normally a high vacuum. The resulting structure forms a sealed enclosure at a high vacuum level.
Various techniques can be utilized to enhance the sealing process of the invention. For example, venting slots can be provided along the first edge of the wall to assist in removing gases from the enclosure as the first plate structure is sealed to the wall. A positioning structure, such as a plurality of posts, can be employed to hold the plate structures in a fixed position relative to each other before using gap jumping to seal the first plate structure to the wall. The positioning structure is preferably located outside the wall and thus has no effect on the sealed enclosure.
The wall can have a profile in two distinct portionsxe2x80x94e.g., generally shaped like a xe2x80x9cTxe2x80x9d or an inverted xe2x80x9cLxe2x80x9dxe2x80x94in which one of the portions is wider than the other. A surface of the wider portion forms the wall""s first edge. During local transfer of energy to the wall along its first edge, the wider portion compresses along its width to facilitate gap jumping.
When a light source that produces a beam at wavelengths that fall into multiple different wavelength domains is employed to perform the subsequent energy-transferring step in which sealing of the first plate structure to the wall is completed, the same light source can be utilized concurrently to transfer energy locally to material of the first plate structure along its sealing area in order to raise that material to a temperature close to the melting temperature of the wall along its first edge. In this case, the beam energy in one of these wavelength domains is transferred locally to material of the wall along its first edge while the beam energy in another of the wavelength domains is simultaneously transferred locally to material of the first plate structure along its sealing area. Locally heating both the first plate structure and the wall in this way provides stronger bonding at the seal interface and thus increases the hermeticity of the seal.
The laser employed in performing local energy transfer in the sealing processes of the invention preferably generates a laser beam of selected non-circular, typically rectangular, cross section. Due to the mechanics of how energy is transferred to a sealing area, the rectangular cross,section of the laser beam causes the light energy to be distributed more uniformly across the sealing area. The creation of bubbles is substantially inhibited in the sealed material, thereby also producing a stronger seal. In short, the invention provides a highly consistent, effective technique for hermetically sealing a flat-panel device, especially a flat-panel display of the CRT type.